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Abstract:

The present invention provides blowing agent introduction systems and
methods for introducing blowing agent into polymeric foam processing
systems. The methods may involve introducing one or more doses of blowing
agent into polymeric material in an extruder during a plastication period
of a molding cycle. Prior to introduction, a dose is confined in an
accumulator which, for example, has a volume defined between an inlet
valve and an outlet valve. A control system may receive inputs related to
system parameters (e.g., pressure of polymeric material in extruder,
axial position of screw in barrel, whether screw is rotating, and the
like) and based on the inputs coordinates operation of the valves to
confine and introduce the blowing agent dose(s). The introduction systems
may be used in discontinuous plasticating processes, such as injection
molding and blow molding, and can be particularly useful in processes
that use precise quantities of blowing agent.

Claims:

1. A blowing agent introduction system comprising: an accumulator having
an inlet connectable to a source of blowing agent and an outlet
connectable to a blowing agent port of an extruder; a pressure regulating
device positioned between the source of blowing agent and the inlet of
the accumulator, the pressure regulating device designed to control the
pressure of blowing agent delivered to the accumulator; an outlet valve
associated with the outlet of the accumulator; a pressure measuring
device constructed and arranged to measure pressure of polymeric material
in the extruder; and a control system capable of receiving a first input
signal from the pressure measuring device representative of the pressure
of polymeric material in the extruder and sending a first output signal
to open the outlet valve when the pressure of blowing agent delivered to
the accumulator is greater than the pressure of polymeric material in the
extruder.

2. The system of claim 1, wherein the accumulator has a substantially
fixed volume.

3. The system of claim 1, further comprising a second pressure measuring
device constructed and arranged to measure pressure of blowing agent in
the accumulator.

4. The system of claim 1, wherein the control system sends a second
output signal to the pressure regulating device to set the pressure to
control the pressure of blowing agent delivered to the accumulator to a
value greater than the pressure of polymeric material in the extruder.

5. The system of claim 1, wherein the pressure regulating device is
designed to be manually operated by a user to control the pressure of
blowing agent delivered to the accumulator to a value greater than the
pressure of polymeric material in the extruder.

6. The system of claim 1, wherein the volume of the accumulator is
adjustable between injection molding cycles.

7. The system of claim 1, further comprising a temperature measuring
device associated with the accumulator.

8. The system of claim 1, further comprising an inlet valve associated
with the inlet of the accumulator.

9. A system comprising: an extruder including a screw designed to rotate
within a barrel to plasticate polymeric material during a plastication
period of a molding cycle, the extruder having a blowing agent port; an
accumulator having an inlet connectable to a source of blowing agent and
an outlet connectable to the blowing agent port; an outlet valve
associated with the outlet of the accumulator; and a control system
capable of receiving an input signal representative of the start of the
plastication period and sending an output signal to open the outlet
valve.

10. The system of claim 9, wherein the control system is capable of
receiving an input signal representative of an axial position of the
screw in the extruder during a plastication period.

11. The system of claim 9, wherein the control system is capable of
receiving an input signal representative of a time after the start of
screw rotation in the extruder.

12. The system of claim 9, wherein the system further comprises an
injection mold connected to an outlet of the extruder.

13. The system of claim 9, wherein the accumulator has a substantially
fixed volume.

14. A blowing agent introduction system comprising: an accumulator
designed to confine a dose of blowing agent, the accumulator having an
inlet connectable to a source of blowing agent and an outlet connectable
to a blowing agent port of a polymer processing apparatus, wherein the
system is constructed and arranged to determine the mass of the dose of
blowing agent in response, at least in part, to the pressure of the dose
of blowing agent, and to introduce the dose of blowing agent into
polymeric material in the polymer processing apparatus as a result of the
pressure of the first dose being greater than the pressure of the
polymeric material in the polymer processing apparatus in the vicinity of
the blowing agent port.

15. The blowing agent introduction system of claim 14, wherein the
accumulator has a substantially fixed volume.

16. The blowing agent introduction system of claim 14, wherein the system
is designed to introduce a series of doses having a known mass into
polymeric material in a polymer processing apparatus.

17. The blowing agent introduction system of claim 14, wherein the system
is constructed and arranged to determine the mass of the dose of blowing
agent in response, at least in part, to the temperature and volume of the
dose of blowing agent.

Description:

RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent application Ser.
No. 11/544,295, filed Oct. 6, 2006, which is a divisional of U.S. patent
application Ser. No. 10/281,891, filed Oct. 28, 2002, which are
incorporated herein by reference in their entireties.

BACKGROUND

[0002] 1. Field of the Invention

[0003] The present invention relates generally to polymer foam processing
and, more particularly, to systems and methods for introducing blowing
agent into polymeric material in a polymeric foam process.

[0004] 2. Description of the Related Art

[0005] Polymeric materials are processed using a variety of techniques.
Many techniques employ an extruder which includes a polymer processing
screw that rotates within a barrel to plasticate polymeric material. Some
processing techniques, such as injection molding and blow molding, may be
discontinuous. That is, during operation, the screw does not plasticate
polymeric material continuously. Discontinuous processes may have
repetitive cycles which include a plastication period, in which the screw
rotates and polymeric material is accumulated, followed by an injection
(or ejection) period, in which the screw does not rotate and the
accumulated polymeric material is injected into a mold (or ejected
through a die).

[0006] Polymeric foam materials, including microcellular materials, may be
processed by introducing a physical blowing agent into the polymeric
material within the extruder through a blowing agent port in the barrel.
Many conventional blowing agent introduction systems introduce blowing
agents continuously into the polymeric material within the barrel. In
discontinuous processes, including certain injection molding and blow
molding processes, such continuous introduction systems may cause a lack
of control over the percentage of blowing agent injected into the
polymeric material and may lead to an uneven distribution of the blowing
agent in the polymeric material. In particular, the polymeric material in
the vicinity of the blowing agent port, when the screw ceases to
plasticate polymeric material, may contain higher amounts of blowing
agent because of its increased residence time in proximity to the blowing
agent injection port. The uneven distribution of blowing agent may result
in viscosity variations within the polymeric material in the extruder
which can cause output inconsistencies and other problems. Such effects
may generally reduce control over the process and may narrow the
processing window.

[0007] In some polymer processes, including some discontinuous processes,
such conventional blowing agent introduction systems may be adequate.
However, in other processes, such as discontinuous processes that require
relatively precise control over blowing agent introduction, the
conventional introduction systems may impair the process for one or more
of the reasons described above. In particular, certain processes for
producing small molded articles and/or microcellular foam articles may be
adversely affected if the blowing agent is not precisely controlled.

[0009] In one aspect, the invention includes a blowing agent introduction
system. In one set of embodiments, the blowing agent introduction system
includes an accumulator having an inlet connectable to a source of
blowing agent and an outlet connectable to a blowing agent port of an
extruder. The system further includes a pressure regulating device
positioned between the source of blowing agent and the inlet of the
accumulator. The pressure regulating device is designed to control the
pressure of blowing agent delivered to the accumulator. The system
further includes a pressure measuring device constructed and arranged to
measure pressure of polymeric material in the extruder. The system
further includes a control system capable of receiving a first input
signal from the pressure measuring device representative of the pressure
of polymeric material in the extruder and sending a first output signal
to the pressure regulating device to control the pressure of blowing
agent delivered to the accumulator to a value greater than the pressure
of polymeric material in the extruder.

[0010] In another set of embodiments, the invention includes a blowing
agent introduction system. The system includes an accumulator having an
inlet connectable to a source of blowing agent and an outlet connectable
to a blowing agent port of an extruder. The system further includes a
pressure regulating device positioned between the source of blowing agent
and the inlet of the accumulator. The pressure regulating device is
designed to control the pressure of blowing agent delivered to the
accumulator. The system further includes an outlet valve associated with
the outlet of the accumulator, and a pressure measuring device
constructed and arranged to measure pressure of polymeric material in the
extruder. The system further includes a control system capable of
receiving a first input signal from the pressure measuring device
representative of the pressure of polymeric material in the extruder and
sending a first output signal to open the outlet valve when the pressure
of blowing agent delivered to the accumulator is greater than the
pressure of polymeric material in the extruder.

[0011] The invention includes a blowing agent introduction system in
another set of embodiments. The system includes an accumulator having a
substantially fixed volume, an inlet connectable to a source of blowing
agent and an outlet connectable to a blowing agent port of an extruder.
The system further includes a control system capable of adjusting
pressure of blowing agent in the accumulator in response to pressure of
polymeric material in the extruder.

[0012] In another aspect, the invention provides a system. The system
includes an extruder including a screw designed to rotate within a barrel
to plasticate polymeric material during a plastication period of a
molding cycle. The extruder has a blowing agent port. The system further
includes an accumulator having an inlet connectable to a source of
blowing agent and an outlet connectable to the blowing agent port. The
system further includes an outlet valve associated with the outlet of the
accumulator, and a control system capable of receiving an input signal
representative of the start of the plastication period and sending an
output signal to open the outlet valve.

[0013] In another aspect, the invention includes a method. In one set of
embodiments, the method includes the steps of plasticating polymeric
material in an extruder, introducing a first dose of blowing agent into
the polymeric material while plasticating the polymeric material in a
first plastication period of a molding cycle, introducing a second dose
of blowing agent into the polymeric material while plasticating the
polymeric material in the first plastication period of the molding cycle,
and injecting a mixture of blowing agent and polymeric material into a
mold.

[0014] In another set of embodiments, the invention includes a method of
introducing blowing agent into polymeric material. The method includes
the steps of plasticating polymeric material in an extruder, and
sequentially introducing blowing agent into the polymeric material during
a plastication period of a first molding cycle at a first rate, a second
rate, and a third rate. The second rate has a minimum value that is less
than 50% of the smaller of the maximum values of the first and third
rates.

[0015] In another set of embodiments, the invention includes a method of
introducing blowing agent into polymeric material. The method includes
the steps of measuring pressure of polymeric material in an extruder and
controlling pressure of blowing agent in an accumulator to a value
between about 50 psi and about 1000 psi greater than the pressure of
polymeric material in the extruder. The method further includes
introducing blowing agent from the accumulator into the polymeric
material in the extruder.

[0016] Other advantages, novel features, and objects of the invention will
become apparent from the following detailed description of non-limiting
embodiments of the invention when considered in conjunction with the
accompanying drawings, which are schematic and which are not intended to
be drawn to scale. In the figures, each identical or nearly identical
component that is illustrated in various figures typically is represented
by a single numeral. For purposes of clarity, not every component is
labeled in every figure, nor is every component of each embodiment of the
invention shown where illustration is not necessary to allow those of
ordinary skill in the art to understand the invention. In cases where the
present specification and a document incorporated by reference include
conflicting disclosure, the present specification shall control.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Non-limiting embodiments of the present invention will be described
by way of example with reference to the accompanying drawings in which:

[0018]FIG. 1 schematically illustrates a blowing agent introduction
system used to introduce blowing agent into an extruder of an injection
molding system;

[0019]FIG. 2 schematically illustrates another blowing agent introduction
system used to introduce blowing agent into an extruder of an injection
molding system;

[0020]FIG. 3 schematically illustrates another blowing agent introduction
system used to introduce blowing agent into an extruder of an injection
molding system;

[0021]FIG. 4 schematically illustrates a section of an extruder barrel
including multiple blowing agent ports connected to a conduit of a
blowing agent introduction system;

[0022]FIG. 5 schematically illustrates a valve assembly of the blowing
agent introduction system positioned in an extruder barrel;

[0023] FIGS. 6A and 6B are copies of SEM photos that show representative
cross-sections of parts produced in Example 3;

[0024] FIGS. 7A and 7B are copies of SEM photos that show representative
cross-sections of parts produced in Example 3; and

[0025] FIGS. 8A and 8B are copies of SEM photos that show representative
cross-sections of parts produced in Example 3.

DETAILED DESCRIPTION

[0026] The present invention provides blowing agent introduction systems
and methods for introducing blowing agent into polymeric foam processing
systems. The methods may involve introducing one or more doses of blowing
agent into polymeric material in an extruder during a plastication period
of a molding cycle. Prior to introduction, a dose is confined in an
accumulator which, for example, has a volume defined between an inlet
valve and an outlet valve. As described further below, a control system
may receive inputs related to system parameters (e.g., pressure of
polymeric material in extruder, axial position of screw in barrel,
whether screw is rotating, and the like) and based on the inputs
coordinates operation of the valves to confine and introduce the blowing
agent dose(s). The introduction systems may be used in discontinuous
plasticating processes, such as injection molding and blow molding, and
can be particularly useful in processes that use precise quantities of
blowing agent.

[0027] Referring to FIG. 1, one illustrative embodiment of a blowing agent
introduction system 10 according to the invention is shown connected to
an extruder 12 of an injection molding system 14. Introduction system 10
introduces blowing agent from a source 22 through a blowing agent port 24
into polymeric material in extruder 12. Conduit 26 connects the blowing
agent source to various components of the introduction system and to the
blowing agent port. Downstream of source 22, the introduction system
includes a pump 85 to increase the pressure of blowing agent, a high
pressure tank 83 to store blowing agent, and a pressure regulating device
(e.g., a pressure reducing regulator 81) to regulate blowing agent
downstream of the device to a fixed value. The introduction system also
includes an accumulator 16 having a volume defined between an inlet valve
28 and an outlet valve 30. In the embodiment of FIG. 1, the accumulator
includes a chamber 20 which defines the majority of the volume of the
accumulator. In other embodiments, the chamber may not define the
majority of the accumulator volume. Inlet valve 28 is positioned between
the blowing agent source and the accumulator to permit/prevent the flow
of blowing agent into the accumulator and outlet valve 30 is positioned
between the accumulator and the blowing agent port to control the flow of
blowing agent to the blowing agent port. In some cases and as shown, it
may be preferable to position the outlet valve proximate to port 24. A
control system 29 which, as described further below, may receive input
signals from an injection molding controller 31 and/or introduction
system 10 and send output signals to open and close valves 28, 30, thus,
controlling the operation of the introduction system. In systems that do
not include an injection molding controller 31, the control system 29 may
receive inputs from components of the injection molding system.

[0028] It should be understood that the introduction system may have a
variety of different configurations in other embodiments of the present
invention. For example, as shown in FIG. 2 and described further below,
the accumulator may not include a chamber. Also, in certain embodiments,
the introduction system may not include certain components such as a pump
or a high pressure tank. In some embodiments, the pressure regulating
device may be a back pressure regulator as shown in FIG. 2 and described
further below.

[0029] Injection molding system 14 may be any suitable type known in the
art. Examples of suitable injection molding systems have been described,
for example, in International Publication No. WO 98/31521, entitled
"Injection Molding of Microcellular Material," by Pierick et. al. which
is incorporated herein by reference. In the illustrative embodiment of
FIG. 1, the injection molding system includes extruder 12 having an
outlet 33 fluidly connected to an injection mold 32. The extruder
includes a polymer processing screw 34 that is mounted within barrel 36.
Polymer processing screw may be rotated and moved axially in a downstream
direction, for example, by a motor 37. Polymeric material, typically in
pelletized form, is fed into barrel 36 from a hopper 38 through an
orifice 40. Screw 34 and barrel 36 define therebetween a polymer
processing space 42 in which polymeric material is conveyed in a
downstream direction 44 during the plastication period of the cycle by
the rotation of the screw. Blowing agent port 24 is formed in the barrel
to permit introducing of blowing agent into the polymeric material in the
polymer processing space. Barrel 36 may be equipped with temperature
control units 46 at selective positions along its axial length.
Temperature control units 46 may be used to heat the barrel, for example
to facilitate melting of pelletized polymeric material, or cool the
barrel, for example to increase the viscosity of the polymeric melt.
Extruder 12 may also include measurement instruments (not shown) such as
thermocouples and pressure transducers to monitor, respectively, the
temperature and pressure of the polymeric material at various locations
along the length of the barrel.

[0030] The operation of the blowing agent introduction system may be
coupled to the plasticating period of the molding cycle. At the beginning
of a molding cycle, screw 34 is positioned at a downstream end of barrel
36 as shown in FIG. 1 and, typically, a quantity (or "dose") of blowing
agent is confined at a relatively high pressure in accumulator 16. Screw
34 begins to rotate within barrel 36 to plasticate polymeric material and
convey the polymeric material in a downstream direction in the polymer
processing space. At the start of, or at any point during plastication,
outlet valve 30 may be opened by control system 29. The control system,
for example, may open the outlet valve in response to a specified
condition as described further below. For example, the control system may
open the outlet valve when blowing agent pressure in the accumulator is
greater than the pressure of polymeric material in the extruder. When the
outlet valve is opened, the blowing agent in the accumulator expands,
thus, passing through the blowing agent port and entering the polymeric
material to form a mixture of polymeric material and blowing agent in the
polymer processing space.

[0031] After the dose is introduced, the outlet valve is closed.
Typically, the inlet valve is then opened, for example by a signal from
the control system, which allows blowing agent to flow into the
accumulator. The inlet valve may then be closed to confine a dose of
blowing agent in the accumulator between the closed inlet and outlet
valves. In some processes, as described further below, multiple blowing
agent doses may be introduced during a single plastication period.

[0032] After the dose is introduced, the polymeric material and blowing
agent mixture is conveyed downstream by the rotating screw and
accumulated in a region 52 within the barrel downstream of the screw. The
accumulated mixture creates a pressure that causes the screw to retract
axially in an upstream direction in the barrel. Typically, plastication
continues until a desired quantity of mixture (or shot) is accumulated.
In some embodiments, as described further below, the amount of blowing
agent introduced during a plastication period may be controlled to create
a shot having the desired weight percentage of blowing agent. For
example, the amount of blowing agent may be controlled by introducing
multiple blowing agent doses in a single plastication period or
controlling the dose size (e.g., by accumulator volume and/or pressure of
blowing agent in accumulator and/or temperature of blowing agent in the
accumulator).

[0033] After a shot has been accumulated in region 52, screw 34 ceases to
rotate and to retract. Then, screw 34 is moved axially in a downstream
direction (indicated by 44 in FIG. 1) to inject the accumulated shot
through outlet 33 of the extruder and into the mold. A valve 54
associated with the outlet of the extruder may be opened to permit the
mixture to flow into the mold. The mixture of polymeric material and
blowing agent is cooled in the mold, after which the mold is opened to
produce an injection molded foam article.

[0034] As noted above, control system 29 may receive input signals from
components of the molding system and/or send output signals to open
and/or close valves 28, 30 and to control the operation of other
components (e.g., pressure regulating device 81) of the introduction
system.

[0035] In some process, control system 29 receives input signals from a
temperature measuring device 94 positioned within accumulator 20. As
described further below, the controller may determine the mass of blowing
agent confined in the accumulator (i.e., dose size) from the blowing
agent temperature and other inputs (e.g., blowing agent pressure). The
temperature measuring device as shown is positioned within chamber 20,
although it should be understood that it may be positioned elsewhere in
the accumulator.

[0036] In some embodiments, the control system receives an input signal
from a pressure measuring device 86 related to the pressure of polymeric
material in the polymer processing space in the vicinity of the blowing
agent port. In the embodiment of FIG. 1, pressure measuring device 86 is
inserted through an instrument port 88 formed in the barrel proximate the
blowing agent port. For example, instrument port 88 may be positioned
within about 12 inches, within about 5 inches, or within about 1 inch
from blowing agent port 24. In other embodiments, the pressure measuring
device may indirectly measure the pressure of polymeric material in the
vicinity of the blowing agent port, for example, by measuring or
determining a pressure elsewhere in the system (e.g., hydraulic pressure
at back of the screw) and inferring or calculating the pressure of the
polymeric material.

[0037] In some cases, the control system may send an output signal to a
component of the introduction system to control the pressure of blowing
agent in accumulator 16. For example, the control system may send an
output signal that controls the pressure of blowing agent in the
accumulator in response to the input signal from pressure measuring
device 86. The pressure of blowing agent in the accumulator may be
adjusted to a desired value by sending an output signal, for example, to
a pressure regulating device (e.g., pressure reducing regulator 81, FIG.
1; back pressure regulator 19, FIG. 2). The pressure regulating device,
in turn, fixes the pressure of blowing agent delivered to the accumulator
when inlet valve 28 is opened. In other cases, the pressure of blowing
agent in the accumulator may be adjusted by sending an output signal to
other components capable of fixing the pressure delivered to the
accumulator.

[0038] The control system can set the pressure of the blowing agent
supplied to the accumulator at a value greater than the pressure of
polymeric material in the extruder in the vicinity of the blowing agent
port. Typically, the control system sets the blowing agent pressure
greater than the polymeric material pressure by at least a critical
amount. The amount is selected so as to ensure there is sufficient
driving force to introduce the dose of blowing agent and to maintain
blowing agent pressure greater than the polymeric material pressure
throughout introduction even when the polymeric material pressure
fluctuates in the extruder. In some cases, the control system may send an
output signal that sets the blowing agent pressure in the accumulator at
least 50 psi greater than the polymeric material pressure in the vicinity
of the blowing agent port. In other cases, the control system may send an
output signal that sets the blowing agent pressure in the accumulator at
least 200 psi, or even at least 400 psi, greater than the polymeric
material pressure in vicinity of the blowing agent port. Higher pressure
differences may be desirable in certain processes such as those that are
characterized by having relatively high pressure fluctuations in the
extruder. Higher pressure differences may also increase the rate of
blowing agent introduction.

[0039] In some embodiments, the control system sets the pressure of the
blowing agent supplied to the accumulator greater than the pressure of
polymeric material in the extruder but less than a critical amount. If
the pressure difference between the pressure of blowing agent in the
accumulator and the pressure of polymeric material in the extruder is too
great than the molding process may be negatively effected. For example,
if the pressure difference is too high than the blowing agent may not be
properly mixed in the extruder which can effect the quality of molded
parts. Furthermore, if the pressure difference is too high than blowing
agent may cause backflow in the extruder. These effects may impair
process stability. In some processes, the control system may send an
output signal that sets the blowing agent pressure in the accumulator
greater than the polymeric material pressure in the vicinity of the
blowing agent port by a value less than 1000 psi greater than the
polymeric material pressure. In other processes, the control system sets
the blowing agent pressure in the accumulator greater than the polymeric
material pressure in the vicinity of the blowing agent port by a value
less than 500 psi greater than the polymeric material pressure. The
specific pressure difference may depend on the process. For example, some
processes may be more sensitive to higher pressure differences.

[0040] It should be understood that in certain processes, the controller
sets the blowing agent pressure in the accumulator greater than the
polymeric material pressure within a critical range. The lower limit of
the range may be any of the values disclosed above (e.g., 50 psi, 200
psi, and 400 psi) and the upper limit of the range may be any of the
values disclosed above (e.g., 500 psi or 1000 psi). For example, the
controller may set the blowing agent pressure in the accumulator between
50 psi and 1000 psi greater than the pressure in the polymeric material
in the extruder in the vicinity of the blowing agent port. In other
cases, the controller sets the blowing agent pressure in the accumulator
between 50 psi and 500 psi greater than the pressure of the polymeric
material in the extruder in the vicinity of the blowing agent port.

[0041] It should also be understood, however, that in some embodiments of
the invention the control system does not need to send an output signal
to a component of the introduction system to control the pressure of
blowing agent in the accumulator. In some embodiments, a user may
manually set the pressure delivered to the accumulator, for example, by
manually setting the pressure regulator. In other embodiments, the
blowing agent may be provided to the accumulator (e.g., by source 22) at
a sufficient pressure without the need for adjustment.

[0042] Once the desired pressure value of blowing agent in the accumulator
is achieved, the control system may send an output signal that opens
outlet valve 30 to introduce blowing agent into the polymeric material.
In some cases, the introduction system includes a pressure measuring
device 90 to confirm when the blowing agent pressure in the accumulator
has reached its desired value. In these cases, pressure measuring device
90 may send input signals to the control system. In response to these
input signals, the control system sends an output signal to open the
outlet valve when the appropriate pressure condition has been achieved.

[0043] However, it should be understood that in some embodiments, the
introduction system may not include a device that measures pressure in
the accumulator. In these embodiments, the pressure in the accumulator is
assumed to be a certain value, for example, the value set by the pressure
regulating device. Elimination of pressure measuring device 90 may
advantageously simplify the design of the introduction system in certain
cases.

[0044] When pressure measuring device 90 is present, the device may be
inserted via an instrument port in chamber 20 of the accumulator. It
should also be understood that the pressure measuring device 90 may be
inserted into other regions (e.g., conduit 26) of the accumulator. In
other embodiments, pressure measuring device 90 may indirectly measure
the pressure of the blowing agent port, for example, by measuring or
determining a pressure elsewhere in the system and inferring or
calculating the pressure of the blowing agent.

[0045] In another set of embodiments, control system 29 may send an output
signal to open the outlet valve in response to an input signal from
injection molding controller 31 (or from components of the injection
molding system) related to the onset of the plastication period of the
molding cycle (e.g., based on axial position of the screw or time after
onset of screw rotation) instead of in response to an input signal
related to the pressure of polymeric material as described above. Thus,
these processes may not require direct measurement of the polymeric
material pressure. It should be understood that, in some processes, the
control system 29 may receive input signals from the injection molding
controller 31 related to the onset of the plastication period and receive
input signals related to the pressure of polymeric material in the
extruder.

[0046] In some embodiments when the control system receives an input
signal related to the onset of the plastication period, the control
system receives an input signal related to the axial position of the
screw within the barrel, for example, as the screw retracts during
plastication. The control system may, in turn, send an output signal that
opens the outlet valve when the screw retracts to a given axial position
in the barrel. The retraction distance prior to the outlet valve opening
is generally long enough to ensure that the pressure in the extruder has
relatively stabilized. In some cases, the control system sends an output
signal that opens the valve when the screw has retracted at least 0.5
inches from its original position. In other cases, the control system
sends an output signal that opens the valve when the screw has retracted
at least 0.1 inches. It should be understood that the screw retraction
distance at which the output signal is sent depends on a variety of
factors (e.g., total retraction distance throughout cycle) and other
retraction distances may be suitable.

[0047] In other embodiments, the control system receives an input signal
related to the onset of screw rotation. The control system may, in turn,
send an output signal that opens the outlet valve when the screw begins
to rotate or, more typically, a specified time period thereafter. The
specified time period is generally long enough to ensure that pressure in
the extruder has relatively stabilized. In some cases, the control system
sends an output signal that opens the valve at least 0.5 seconds, or at
least 1.0 seconds, after the onset of screw rotation. Other times are
also suitable.

[0048] The control system may also send an output signal to close outlet
valve 30. The outlet valve is typically closed after the dose of blowing
agent has been introduced into the polymeric material. That is, the
outlet valve is typically closed after blowing agent pressure in the
accumulator has decreased to approximately equal the pressure of
polymeric material in the extruder. However, it should be understood that
in some cases, the outlet valve may be closed prior to the blowing agent
pressure becoming approximately equal to the pressure of polymeric
material in the extruder.

[0049] In certain processes, the control system sends an output signal to
close the outlet valve a specified time period after the outlet valve has
been open. That is, the control system opens the valve for a specified
time period. The specified time period may, for example, be 0.5 seconds
or more. In other cases, the specified time period may be 1.0 seconds or
more; in other cases, the specified time period may be 5.0 seconds or
more. Other specified time periods are also suitable. The specified time
period depends on a variety of factors including dose size, number of
doses, and plastication period time, amongst others.

[0050] It should be understood that in some processes, the control system
may send an output signal to close the outlet valve based on other system
parameters. For example, the control system may send an output signal to
close the outlet valve because of a pressure condition in the extruder or
in the accumulator.

[0051] The control system also typically controls the operation of inlet
valve 28. Generally, the inlet valve is closed when outlet valve 30 is
open. When the outlet valve is closed, the control system may send a
signal that opens the inlet valve. As described above, in some processes,
the control system also sends a signal to regulate or control blowing
agent pressure supplied to the accumulator (e.g., by use of a pressure
regulating device). In these cases, the inlet valve may be opened after
the desired supply pressure is reached. Once the inlet valve is open,
blowing agent may flow into the accumulator. Once the desired dose has
been accumulated, the inlet valve may be closed by the control system to
confine the blowing agent. In some cases, the inlet valve may be left
open even after the dose has been accumulated and then closed immediately
prior to opening the outlet valve.

[0052] As noted above, certain processes may introduce more than one dose
of blowing agent during a single plastication period. In one set of
embodiments, control system 29 sends multiple open/close signals to
valves 28 and 30 during each plastication period, allowing several doses
of blowing agent to be delivered. For example, the control system may
cause at least 2, 3, 5, 7, 10, 15, or 20 doses of blowing agent to be
delivered during a given plastication period. The number of doses
introduced during a single plastication period depends on a number of
factors including the desired weight percentage of blowing agent in a
shot, the shot size, and the dose size. It should be understood that
certain processes may utilize a single dose or any other suitable number
of doses.

[0053] Introducing multiple doses during a given plastication period may
be advantageous in certain processes. For example, multiple doses allows
for increased control over the total amount of blowing agent introduced
into the polymeric material in a given shot. The increased control
arises, in part, from using higher accumulator pressures which reduces
inaccuracies resulting from polymeric material pressure variations. In
addition, higher accumulator pressures result in improved blowing agent
introduction into the polymeric material. The increased control enables
precise quantities of blowing agent to be introduced in a given shot.
This precision, for example, may be important in certain processes that
produce shots having relatively small amounts of blowing agent (e.g.,
less than about 2% of CO2 or less than 0.1% of N2), and/or
produce relatively small molded articles (e.g., less than 15 g) and/or
produce microcellular foam articles. Introducing multiple doses
throughout a plastication period may also increase the uniformity of
blowing agent distribution within a shot.

[0054] It should be understood that the introduction system and processes
of the invention may be used in connection with a wide variety of
processes.

[0055] The desired blowing agent percentage in a shot depends upon the
particular process and the blowing agent type. The desired blowing agent
percentage may, for example, be less than about 5% by weight of polymeric
material and blowing agent mixture. In embodiments that utilize CO2
as a blowing agent, the level may be less than about 4% and, in others,
less than about 2% by weight of polymeric material and blowing agent
mixture. When N2 is used as the blowing agent, for example, the
level may be less than about 0.5%, less than about 0.3%, or less than
about 0.1% by weight of polymeric material and blowing agent mixture. As
noted above, the introduction system of the invention may be particularly
well suited to forming blowing agent mixtures at low weight percentages
of blowing agent.

[0056] The shot size also depends upon the particular process and, for
example, may be between about 1 g and about 200 g. As noted above, the
introduction system may be particularly well suited to introduce blowing
agent into relatively small shot sizes, for example, shot sizes of less
than 15 g. In some cases, the shot sizes may be between about 1 g and 15
g.

[0057] The dose size is typically measured by mass. The dose size is the
mass of blowing agent confined in the accumulator that is introduced into
the polymeric material. As described above, multiple doses may be
introduced in a single plastication cycle. In cases where multiple doses
are introduced, the total amount of blowing agent in the shot is the
number of doses multiplied by the dose size.

[0058] The dose size depends on the particular process, as well as, the
volume of the accumulator, and the pressure and temperature of the
blowing agent in the accumulator. Typical dose sizes, for example, may be
between about 0.005 g and about 0.1 g. In some cases, the dose size may
be less than less than 0.1 g; in other cases less than 0.05 g; and, in
other cases less than 0.005 g. Smaller dose sizes may be used, for
example, for increased precision. It should be understood that other dose
sizes are also possible.

[0059] In one set of embodiments, control system 29 may control the total
amount of blowing agent introduced into a shot and/or the number of
blowing agent doses introduced into a shot and/or the size of the dose.
The control system may also receive manual inputs of the desired blowing
agent percentage in the mixture of polymer and blowing agent and/or the
desired dose size and/or the desired shot size. The control system may
process the inputs and sends an output signal to control the introduction
of blowing agent into the mixture so as to form a mixture having the
desired blowing agent percentage.

[0060] In some embodiments, as described above, introduction system 10 may
utilize control system 29 to receive inputs from pressure measuring
device 90 and a temperature measuring device 94 related, respectively, to
the pressure and temperature of the blowing agent in the accumulator. The
control system may also receive manual inputs related to the selected
fixed mass of the blowing agent to be confined in the accumulator; or,
alternatively, may calculate the selected mass of the blowing agent to be
confined in the accumulator from other inputs. The control system may be
programmed to deliver one or more shot sizes, or a desired blowing agent
percentage of the blowing agent and polymeric material mixture. Control
system 29 may also be configured to calculate the volume and/or mass of
blowing agent to be confined in the accumulator using the Ideal Gas law
or other phase equilibria relation, from the inputs of blowing agent
temperature, blowing agent pressure, and selected mass of the blowing
agent.

[0061] The volume of accumulator 16 is defined between the inlet valve and
the outlet valve. In some cases, the volume of the accumulator may be
partially defined within the valve assemblies. The accumulator has a
substantially fixed or fixed volume in some embodiments. As used herein,
a fixed volume refers to a volume that is constant when blowing agent
from the accumulator is introduced into the polymeric material and/or
constant during the plastication period of a molding cycle. That is, in
these embodiments, the accumulator does not include a wall or plunger
assembly that moves when blowing agent is introduced and/or during
plastication. Elimination of the movable wall or plunger assembly may be
advantageous in certain cases because such designs can complicate
construction and operation.

[0062] In some embodiments, the accumulator has a volume that is fixed
during blowing agent introduction and plastication, but can be adjusted
between molding cycles. These types of accumulators also have fixed
volumes as defined herein. In one set of embodiments, the accumulator may
be adjusted, for example, by adjusting a piston or a screw between cycles
that controls the volume of the accumulator, or by replacing the
accumulator with another accumulator having a different size.

[0063] It should also be understood that, in other embodiments, the
accumulator may not have a fixed volume and may utilize movable walls
and/or plungers.

[0064] The volume of the accumulator depends on the specific process and,
in particular, on the desired dose size. In some cases, accumulator 16
may have a volume of less than about 50 cm3. In other cases, the
accumulator volume may be less than about 10 cm3, less than about 1
cm3, less than about 0.1 cm3, or less than about 0.01 cm3.

[0065] In one set of embodiments, accumulator 16 includes a chamber 20
having a cross-sectional area greater than that of the conduit. The
chamber may be in series with the fluidic pathway between valves 28 and
30, as shown in FIG. 1; or, the chamber may have any configuration such
that it remains in fluidic communication with valves 28 and 30, as
illustrated in FIG. 3. The chamber may have any size or shape necessary
for producing a desired volume and containing a pressurized gas or fluid,
for example, rectangular, spherical, or cylindrical. The chamber may be
any of the kind known in the art able to contain a blowing agent. For
example, the chamber may be made of a suitable material for containing a
pressurized gas, liquid, and/or supercritical fluid, such as a metal
tube, preferably made of stainless steel. The selection of configurations
or volumes for the accumulator can be determined by those of ordinary
skill in the art with the benefit of the present invention.

[0066] In another set of embodiments, accumulator 16 may be defined by
conduit existing between valves 28 and valve 30, as illustrated in FIG.
2.

[0067] Valves 28, 30 may be any device in the art that selectively permits
flow of pressurized gas, liquefied gas, or supercritical fluid
therethrough in one configuration, and prevents flow of these fluids
therethrough in another configuration. Suitable types of valves include
solenoid, spool or other equivalent valves, and they may be shut-off
valves. In certain cases, the valve may include a valve body or assembly,
which may define at least a portion of the volume of accumulator 16. In
some cases, valves 28 and 30 may be the same type, while in other cases
the valves may be of different types.

[0068] In the illustrative embodiment of FIG. 1, outlet valve 30 is
positioned near (e.g., less than 12 inches, less than 3 inches or less
than 1 inch) or essentially adjacent to blowing agent port 24. In some
cases, this arrangement may be preferred to minimize the volume of
blowing agent that may be confined in the conduit between the outlet
valve and port 24. However, it is to be understood that the position of
the outlet valve may be elsewhere relative to blowing agent port 24 so as
to control the flow of blowing agent to the blowing agent port.

[0069] When positioned near or adjacent blowing agent port 24, outlet
valve 30 may be of the type shown in FIG. 5 and described in commonly
owned, co-pending, U.S. patent application Ser. No. 09/710,756, filed
Nov. 10, 2000, and entitled "Valve for Injection Molding", and commonly
owned, co-pending, U.S. patent application Ser. No. 09/782,673, filed
Feb. 13, 2001, and entitled "Blowing Agent Delivery System", the
disclosures of which are incorporated herein by reference. FIG. 5 shows
outlet valve 30 as part of an injector assembly that is inserted within
the blowing agent port, thus, is essentially adjacent to the port. As
shown, the outlet valve is formed, in part, from a valve body 168 that is
inserted within an injector sleeve 170. The illustrated valve includes a
valve stem 171 which is actuatable, for example by compressed air,
relative to a valve seat 172 to open or close the valve. In the open
position (as shown in FIG. 5), the valve stem is separated from the valve
seat to provide a pathway that permits blowing agent to flow through an
internal passageway 173 of the valve that is connected to conduit 26
(FIG. 1). In the closed position, the valve stem contacts the valve seat
thereby creating a seal that prevents the flow of blowing agent
therepast. Valve body 168 also may optionally include a back-flow valve.
As illustrated, the back-flow valve includes a ball check 174 which is
upwardly biased and held in position by a spring 175, though other valve
constructions may also be used. In other embodiments, the spring is not
required and the ball may move freely in response to the blowing agent
and polymeric material. Typically, when the shut-off valve is open, the
pressure of the blowing agent forces the ball check 174 away from a
sealing surface 176 to provide a pathway for blowing agent flow to port.

[0070] Blowing agent port 24 is formed in barrel 36 of the extruder at a
location where the polymeric material is generally in a fluid state. The
blowing agent port connects the blowing agent introduction system to the
polymeric material in polymer processing space 42. Blowing agent port 24
may be a single port or a plurality of ports arranged in the barrel. When
multiple ports are utilized, ports can be arranged radially about the
barrel or in a linear fashion along the length of the barrel (FIG. 4). As
shown in FIG. 4, an arrangement of ports 24a, 24b, 24c along the length
of the barrel can facilitate injection of blowing agent at a relatively
constant location relative to the screw when the screw moves axially (in
an upstream direction) within the barrel as the mixture of polymeric
material and blowing agent is accumulated. Where radially-arranged ports
are used, a plurality of ports 24 may be placed at the 12:00 o'clock,
3:00 o'clock, 6:00 o'clock and 9:00 o'clock positions about the extruder
barrel, or in any other configuration as desired.

[0071] In FIG. 4, separate shut-off valves 62a, 62b, 62c may be provided
at each blowing agent port 24a, 24b, 24c. Shut-off valves 62a, 62b, 62c
can be individually opened and closed, during the plastication time
period, so as to control injection of blowing agent at desired location
relative to the position of the screw, for example, to ensure that
blowing agent is introduced at blowing agent injection section of the
screw. One or more valves may be opened at the same time. In the
illustrative embodiment, each blowing agent port is connected to the same
blowing agent introduction system. However, it should be understood that
in some embodiments, each blowing agent port may be connected to a single
blowing agent introduction system.

[0072] Blowing agent port 24 may include a single orifice or a plurality
of orifices (178, FIG. 5). In the multi-orifice embodiments, the port may
include at least about 2, and some cases at least about 4, and others at
least about 10, and others at least about 40, and others at least about
100, and others at least about 300, and others at least about 500, and in
still others at least about 700 blowing agent orifices. In another
embodiment, port 24 includes an orifice containing a porous material that
permits blowing agent to flow therethrough and into the barrel, without
the need to machine a plurality of individual orifices.

[0073] In certain preferred embodiments, blowing agent port 24 may be
located at a blowing agent injection section 48 of the screw during the
plastication period of the cycle. The blowing agent injection section of
the screw may include full, unbroken flight paths. In this manner, each
flight, passes or "wipes" the blowing agent port including orifices
periodically, when the screw is rotating. This wiping increases rapid
mixing of blowing agent and polymeric material in the extruder the result
is a distribution of relatively finely divided, isolated regions of
blowing agent in the polymeric material immediately upon injection into
the barrel and prior to any mixing. This promotes formation of a uniform
polymer and blowing agent mixture which may be desired in certain types
of polymeric foam processing including microcellular processing.
Downstream of the blowing agent injection section, the screw may include
a mixing section 50 which has highly broken flights to further mix the
polymer and blowing agent mixture to promote formation of a uniform
mixture within the extruder. A preferred uniform mixture, in some cases,
is a homogeneous, single-phase solution. In some embodiments when forming
microcellular material, as described in International Patent Publication
No. WO 98/31521, a homogeneous, single-phase solution of polymeric
material and blowing agent is accumulated in region 52 and is nucleated
as the charge is injected into the mold.

[0074] Source 22 has a larger volume than accumulator 16. The source may
supply any type of physical blowing agent known to those of ordinary
skill in the art including nitrogen, carbon dioxide, hydrocarbons,
chlorofluorocarbons, noble gases and the like, or mixtures thereof. The
blowing agent may be supplied to the chamber in any flowable physical
state, for example, a gas, liquid, or supercritical fluid. According to
one preferred embodiment, source 22 provides carbon dioxide as a blowing
agent. In another preferred embodiment, source 22 provides nitrogen as a
blowing agent. In certain embodiments, solely carbon dioxide or nitrogen
may be supplied by the source. Blowing agents that are in the
supercritical fluid state after injection into the extruder (and,
optionally, before injection as well), in particular supercritical carbon
dioxide and supercritical nitrogen, are especially preferred in certain
embodiments.

[0075] Conduit 26 may be any of the kind known in the art suitable for
transporting a blowing agent. For example, the conduit may be a tube made
of a suitable material for transporting pressurized gas, liquid, and/or
supercritical fluid, such as a metal tube, preferably made of stainless
steel. The conduits also may be defined by passageways within a block of
material, such as drilled passageways within a block of metal such as
stainless steel. The blowing agent conduits typically have a
cross-sectional diameter between about 0.1 mm and about 1.0 cm, though
other dimensions are possible. The length and configuration of conduits
26 are not constrained and generally depend upon factors such as
available manufacturing space, and the layout of the blowing agent
introduction system and injection molding system. In some embodiments, it
may be desirable to reduce or minimize the volume of conduits to ensure
that excess blowing agent confined within the conduit does not
significantly affect the process. The conduit assembly, in some
embodiments and as illustrated, may have one or more branches, for
example, to facilitate connection to accumulator 16 (FIG. 3) or to
provide connection to multiple blowing agent ports (FIG. 4).

[0076] Control system 29 and injection molding controller 31 may be any
suitable type known in the art. Suitable control systems and controllers
have been described, for example, in co-pending, commonly-owned U.S.
patent application Ser. No. 09/826,603, entitled "Method and Apparatus
for Controlling Foam Molding Processing," which was filed on Apr. 5, 2001
and is incorporated herein by reference. Some systems and processes of
the invention do not utilize an injection molding controller.

[0077] Pump 85 may be any suitable type known in the art for increasing
the pressure of blowing agent supplied from the source.

[0078] High pressure tank 83 may also be any suitable type known in the
art for storing blowing agent. Tank 83 has a volume that is less than
source 22 but greater than the volume of accumulator 16. The high
pressure tank is provided to limit the effects of pressure surges into
the accumulator when the pump cycles. Typical pressures in the tank may
be between about 5,000 psi and 10,000 psi (e.g., about 7,000 psi).

[0079] The pressure regulating device may also be any suitable type known
in the art for regulating downstream pressure to a fixed value. In some
cases and as shown in FIG. 1, the pressure regulating device is a
pressure reducing regulator. The pressure at the downstream end of the
pressure reducing regulator may be fixed lower than the pressure at the
upstream end of the device. For example, the pressure at the upstream end
of the device may be about 7,000 psi and the pressure at the downstream
end of the device may be reduced to about 4,000 psi. The fixed lower
pressure is the pressure of blowing agent delivered to the accumulator.

[0080] In other cases, the pressure regulating device may be a back
pressure regulator 19 that is positioned within a branch of the conduit
as shown in FIG. 2. The back pressure regulator provides a fixed upstream
pressure and, for example, may have an outlet vented to the environment.
The fixed upstream pressure is the pressure of blowing agent delivered to
the accumulator.

[0081] As described above, in certain embodiments, the pressure regulating
device receives input signals from the control system and, in response,
fixes the pressure delivered to the accumulator. In these embodiments,
the pressure regulating device must have a suitable design capable of
fixing the pressure delivered to the accumulator in response to input
signals from the control system.

[0082] Though blowing agent introduction system 10 is illustrated in
conjunction with an injection molding system, it is to be understood that
the blowing agent introduction system according to the invention may be
used in conjunction with other polymer processing systems including other
discontinuous systems and, in particular, blow molding systems. Examples
of suitable blow molding systems have been described, for example, in
International Publication No. WO 99/32544 (Anderson et al.) which is
incorporated herein by reference.

[0083] The blowing agent introduction systems may be used to form a wide
variety of molded polymeric material articles and, in particular, molded
foam articles. In some embodiments, a molded microcellular article may be
formed. Suitable microcellular articles have been described in
International Publication No. WO 98/31521 (Pierick et al.) which is
incorporated herein by reference above. In some embodiments, the
microcellular articles have an average cell size of less than 100
microns, in other embodiments less than 50 microns, in other embodiments
less than 25 microns, in other embodiments less than 5 microns, and, in
other embodiments, even smaller cell sizes are achievable.

[0084] The function and advantage of these and other embodiments of the
present invention will be more fully understood from the examples below.
The following examples are intended to illustrate the benefits of the
present invention, but do not exemplify the full scope of the invention.

EXAMPLE 1

[0085] The example illustrates the use of one embodiment of the invention.

[0086] The injection molding machine used was a 55-ton machine having a 20
mm screw. A blowing agent introduction system was connected to the
injection molding machine. The blowing agent introduction system included
an inlet valve, an outlet valve which was incorporated into a blowing
agent injector assembly, and a specific length of conduit to establish
the accumulator volume in which the charge or dose of the blowing agent
was accumulated prior to introduction into the polymer melt, as described
below. The pressure supplied to the accumulator volume was monitored and
controlled using a back-pressure regulator on a T-shaped portion of the
conduit with an outlet vented to air. The temperature of the tubing was
measured using a handheld surface thermocouple device.

[0087] The accumulator volume was calculated as the sum of the injector
assembly volume, the inlet valve volume downstream of the shut-off valve
seat, and the volume of the conduit connecting the valve to the injector.
The injector assembly had a volume of 0.18 cm3, the valve had a
volume of 0.41 cm3, and the tubing had a volume of 0.069 cm3,
for a total volume of 0.659 cm3. The mold in the injection-molding
machine was used to produce a test polypropylene part having a solid part
weight of 1.6 g.

[0088] Using an equation for the density of nitrogen as a function of
temperature and pressure, the nitrogen mass in the control volume
introduced into the polymer melt was calculated based on the initial
pressure in the accumulator volume, and the ending pressure within the
accumulator volume. The initial pressure was a pre-set value controlled
by the back pressure regulator to be greater than the ending pressure.
The apparatus did not include a pressure transducer within the control
volume, so it was assumed that the ending pressure within the control
volume was substantially the same as that of the pressure of the molten
polymer. The polymer melt pressure was monitored using a pressure
transducer located in the molten polymer, adjacent to the injector valve
outlet.

[0089] To introduce the dose of nitrogen, the shut-off valve downstream of
the accumulator was opened for one second. A single dose of nitrogen was
introduced during the plastication period. The shut-off valve upstream of
the accumulator was opened with a time delay of 0.5 seconds after the
shut-off valve downstream of the accumulator was closed to confine an
additional dose of nitrogen.

[0091] The polymer melt pressure at the location for nitrogen injection
was measured to be 2310 psi to 2360 psi during screw recovery. The
nitrogen pressure in the control volume was set to 2550 psi. The
calculated dose of nitrogen was 6 mg, using a dosage of 0.375% per 1.6 g
of plastic weight.

[0092] The process was repeated for one hour with no changes to any
process setting. Approximately 120 foam parts were produced. The parts
throughout the process had similar characteristics. The consistency of
the test parts illustrate the stability of the process, and the ability
to dose small amounts of nitrogen consistently using one embodiment of
the invention.

EXAMPLE 2

[0093] This is an example of an embodiment of the invention using a lower
dosing rate of blowing agent.

[0094] Process parameters in the apparatus in this example were the same
as that of Example 1, but the nitrogen pressure in the accumulator volume
was set to 2475 psi with a polymer melt pressure of 2350 psi. The
calculated amount of nitrogen delivered was approximately 3.8 mg, and the
temperature was about 250° F. The system parameters were not
changed during the run, and the parts were consistently produced for
approximately two hours.

EXAMPLE 3

[0095] The example illustrates the use of another embodiment of the
invention. In particular, this example illustrates the use of the blowing
agent introduction system in connection with a process where the total
part weight required a small percentage of the stroke of the machine. In
this example, the stroke required was about 7% of the total machine
capacity. Typical blowing agent introduction systems cannot be easily
operated with consistency at less than 15% of the machine stroke
capacity.

[0096] The injection molding machine used was a 88-ton machine having a 30
mm screw. The injection molding machine was connected to the blowing
agent injection assembly described in Example 1.

[0097] The mold in the injection-molding machine was used to produce a
U-shaped part having a solid part weight of 6.5 g. The mold had two
cavities which were connected by a centered, cold sprue. The material
used was a glass-filled Nylon 6.

[0098] Using an equation for the density of nitrogen as a function of
temperature and pressure, the nitrogen mass in the control volume
introduced into the polymer melt was calculated based on the initial
pressure in the accumulator volume, and the ending pressure within the
accumulator volume. The initial pressure was a pre-set value greater than
the ending pressure. The apparatus did not include a pressure transducer
within the control volume, so it was assumed that the ending pressure
within the control volume was substantially the same as that of the
pressure of the molten polymer. The polymer melt pressure was monitored
using a pressure transducer located in the molten polymer, adjacent to
the injector valve outlet.

[0099] To introduce the dose of nitrogen, the inlet valve was closed and
the outlet valve downstream of the accumulator was opened for one second.
A single dose of nitrogen was introduced during the plastication period.
The inlet valve of the accumulator was opened with a time delay of 0.3
seconds after the outlet valve of the accumulator was closed to confine
an additional dose of nitrogen.

[0101] The polymer melt pressure at the location for nitrogen injection
was measured to be 3050-3140 psi during screw recovery. The nitrogen
pressure in the control volume was set to four different values.

[0102] The first setting was calculated to deliver 9.3 mg (0.14%
concentration by wgt) of Nitrogen at a accumulator pressure of 3300 psi.
The system was adjusted and approximately 20 cycles were run prior to
collecting parts. The system configuration remained unchanged for
approximately one hour. High quality foam parts were produced. The parts
had a density reduction of about 3.1% relative to the solid plastic. The
parts had a closed cell structure with an average cell size of about 100
microns. FIGS. 6A and 6B are copies of SEM photos that show
representative cross-sections of the parts.

[0103] The second setting was calculated to deliver 13 mg (0.20%
concentration by wgt) of Nitrogen at an accumulator pressure of 3400 psi.
Again parts were collected for one hour and the process and parts were
monitored. High quality parts were produced and the system configuration
was stable. The parts had a density reduction of about 4.1% relative to
the solid plastic. The parts had a closed cell structure with an average
cell size of about 75 microns. FIGS. 7A and 7B are copies of SEM photos
that show representative cross-sections of the parts.

[0104] Two other settings were tested. One was set to deliver 16.5 mg
(0.25% concentration by weight) and the other 18.5 mg (0.28%
concentration by weight) of Nitrogen with accumulator pressure settings
of 3500 and 3600 psi, respectively. High quality parts were produced with
the two additional settings and the system configurations were stable.

[0105] For the 18.5 mg of Nitrogen, the parts had a density reduction of
about 5.3% relative to the solid plastic and a closed cell structure with
an average cell size of about 40 microns. FIGS. 8A and 8B are copies of
SEM photos that show representative cross-sections of the parts.

[0106] Those skilled in the art would readily appreciate that all
parameters and configurations described herein are meant to be exemplary
and that actual parameters and configurations will depend upon the
specific application for which the systems and methods of the present
invention are used. Those skilled in the art will recognize, or be able
to ascertain using no more than routine experimentation, many equivalents
to the specific embodiments of the invention described herein. It is,
therefore, to be understood that the foregoing embodiments are presented
by way of example only and that, within the scope of the appended claims
and equivalents thereto, the invention may be practiced otherwise than as
specifically described. The present invention is directed to each
individual feature, system, or method described herein. In addition, any
combination of two or more such features, systems or methods, if such
features, systems or methods are not mutually inconsistent, is included
within the scope of the present invention.